There are three types of crystal structures of titanium oxide, that is, the tetragonal, high-temperature rutile type, the tetragonal, low-temperature anatase type, and the orthorhombic brookite type. Among these, the rutile type titanium oxide has been used as refractive index regulators because of its high refractive index.
In order to be used as optical materials, the rutile type titanium oxide is required to have transparency when being formed as a coating film as well as a high refractive index. However, the rutile type titanium oxide is generally produced by a solid phase process in which amorphous titanium oxide or anatase type titanium oxide is baked at a high temperature, and the particle diameter disadvantageously increases, thereby transparency is impaired.
Compared with the solid phase process that requires high-temperature baking, a wet process is a method by which fine particles can be easily obtained, because they can be synthesized at a low temperature.
Examples of the method for producing the rutile type titanium oxide by the wet process may include a method in which a titanium salt and a tin compound having the rutile type structure are reacted in a coexistent manner.
As the method that uses the titanium salt and the tin compound, a method is disclosed in which a strong acid salt of titanium and metallic tin are reacted in the presence of hydrogen peroxide to produce an aggregate of a composite colloid of titanium oxide and tin oxide at a temperature from 50° C. to 100° C. (refer to Patent Document 1).
A method is disclosed for producing rutile type titanium oxide fine particles, in which a titanium compound solution coexisting with a tin compound at a Sn/Ti molar ratio of 0.001 to 2 is reacted within a pH range of 1 to 3 at a temperature from room temperature to 100° C. (refer to Patent Document 2).
Another method for obtaining a rutile type titanium oxide sol is a method in which a gel containing titanium atoms is dissolved in hydrogen peroxide and is then reacted with a tin compound or the like; specific examples of the method include a method in which hydrogen peroxide dissolving a hydrated titanium oxide gel and cation-exchanged potassium stannate are mixed and subjected to a heat treatment (refer to Patent Document 3) and a method in which a titanium compound, a tin compound, and ammonia are reacted to form a gel, which is then dissolved in hydrogen peroxide to be subjected to a hydrothermal treatment (refer to Patent Document 4).